Propulsive double-base charges,strand-like and having variable thickness

ABSTRACT

A PROPELLENT CHARGE CONSTITUTED OF SPACED STRANDS OF MATERIAL IS INCREASED IN EFFECTIVELY BY TAPERING THE STRANDS SO THAT THEY DIMINISH IN THICKNESS IN THE DIRECTION OF GAS FLOW. THE SPACES BETWEEN THE STRANDS AND PARTICULARLY THE TAPERING OF THE STRANDS IS SO EFFECTED TO PROVIDE MINIMUM PASSAGE AREAS. THE COMBUSTION TIME OF THE CHARGE IS LESS THAN 1 SECOND AND THE POWDER CHARGES USED HAVE COMBUSTION RATES WHICH ARE SUBSTANTIALLY CONSTANT OVER A LARGE RANGE OF PRESSURES.

Ap 24, 1973 Y. J. G. DUPRAT 3,729,349

PROIULSLVE DOUBLE-BASE CHARGES, STRAND-LIKE AND HAVING VARIABLETHICKNESS Filed Jan. 7, 1971 3 Sheets-Sheet 2 no: V

April 24, 19.73 Y. J. G. DUPRAT ,729,349

, PROPULSIVE DOUBLE-BASE CHARGES, STRAND-LIKE AND HAVING VARIABLETHICKNESS Filed Jan. 7, 1971 3 Sheets-Sheet 5 United States PatentOfiice 3,729,349 Patented Apr. 24, 1973 3,729,349 PROPULSIVE DOUBLE-BASECHARGES, STRAND- LIKE AND HAVING VARIABLE THICKNESS Yves Jean GuyDuprat, 6, rue Gabriel Foucher,

18 Bourges, France Filed Jan. 7, 1971, Ser. No. 104,802 Int. Cl. C06b19/00 US. Cl. 149-2 Claims ABSTRACT OF THE DISCLOSURE A propellentcharge constituted of spaced strands of material is increased inefiectivity by tapering the strands so that they diminish in thicknessin the direction of gas flow. The spaces between the strands andparticularly the tapering of the strands is so effected to provideminimum passage areas. The combustion time of the charge is less than 1second and the powder charges used have combustion rates which aresubstantially constant over a large range of pressures.

BRIEF DESCRIPTION OF THE DRAWING The present invention relates to powderpropellants and an object of the invention is to provide novel powderpropellents whose volume is less than that of known propellents ofequivalent performance:

The maximum gain possible depends on elaborate ballisticcharacteristics.

The principal utilization of the porpellents is in rapidcombustionpropellent units where the combustion period is from a few millisecondsto 1 second.

It is known that a maximum powder load density of a propellent unit(ratio of powder mass to the inside volume of the unit) decreases forany given propellent unit when the combustion time decreases.

It follows that the charges which are the object of the presentinvention, offer the following advantages as compared to the knowncharges:

(1) The volume is reduced for a given mass of powder and givencombustion time;

(2) The mass of powder is increased for a given com bustion time andgiven volume;

(3) The combustion time is shortened for a given mass of powder andgiven volume;

(4) The foregoing properties can be combined, for example, by decreasingcombustion time and increasing powder mass for given volume.

The aforesaid advantages are obtained with an increase of the maximumworking pressure of the propellent unit, and the fluctuations ofspecific impulsion are zero or neglibible in proportion to the overallimpulsion gain.

The propellent charges, according to the invention, consequently entailan improvement in the ratio of impulsion to propellent mass.

One particular example of application is that of antitank missiles forwhich the following conditions obtain:

(a) combustion of propellent in the firing tube so as not to project thepropellent gas jet onto the gunner;

(b) highest possible velocity so as to increase the range of the weaponsystem;

(c) minimum mass for the weapon system (missile and firing tube).

The adoption of a charge according to the invention permits theexpectation of initial velocities of 380 to 400 meters per second for aself-propelled missile of a mass less than 2.5 kg. with total combustioninside a firing tube of a length less than 2 meters.

This is especially interesting because the initial velocity on the onehand is notably increased as against current practice and it is on theother hand supersonic. This allows adding an extra propulsion stage ifnecessary without the drawbacks inherent in crossing the sound barrier.

The propellent charges according to the invention are made up of strandsor filaments of powder whose thickness (their smallest dimension)comprises a straight portion of uniform thickness tapering thereafterfrom the top downward (in the direction of gas discharge flow). Thecharge may conceivably consist of a single strand. The variation ofthicknes is the same for all the strands and defines the duration ofcombustion, subject to the properties of the powder composition used.

The powder strands can be either fiat or coiled strips of various shapeand sizes, or they may be tubular.

The advantages derived from the invention are not the result of theshape of the strands, but to the tapering in thickness and to anarrangement of the charge such that the open spaces, between strands orinside each strand, can in each section satisfy the conditions ofappropriate constrictions in a plane perpendicular to the direction offlow.

This condition is defined as follows: for a pre-determined interval orspace, the ratio of gaseous emission area, upstream in relation to thefilaments traversing the space in question, to the area of passage mustbe less than K/p; K being the constriction (ratio of total emissionsurface area to the surface area of the nozzle passage) and B acoefficient always greater than 1, dependent on the geometry of thecharge and the composition of the powder in use.

By way of nonrestrictive example, the following processes can be used togovern the spacing of the strands; longitudinal gluing of narrow spacertapes over only a small part of the powder area, stamping of smallblisters on the powder strips, use of a built-in metal sheathing.

It is also possible to set up the powder strands with sufficient freedomfor the distribution of pressures on firing to keep the spaces at theiroptimum value.

The powder compositions to be used have the characteristics of stablecombustion in a large pressure area, defined by a maximum pressure P1and a minimum pressure P2. In this same area, the rate of combustionmust not vary greatly. To the pressures P1 and P2, the constrictions Kand K respectively correspond.

Subject to the form of the strands, the powder function (development ofemissive surface in terms of burned thickness) will decrease from aninitial value S to a final value S According to the invention, thevalues S and S must comply with the condition:

s a S2 K2 The propellent nozzle is adapted so that at the maximum itspressure may be equal to P1, the pressure then decreasing to a value notless than P2. Since the speed of combustion is not greatly dependent onthe pressure, the rate of combustion is appreciably equal to that of asteadyemission propellent.

Quite a number of powders possess the necessary characteristics,especially some double-base nitroglycerine-nitrocellulose with a highrate of combustion, corresponding to the most desirable case. a

The following manufacturing processes for the powder strands can be usednonrestrictively.

The strands can be manufactured'as strips by passing sheeting crosswisebetween two rollers spaced to give the required thickness. Two or morestrips can be made at the same time.

It is also possible to manufacture the strips by pressing sheetslengthwise between two drums mounted on a flexible bearing. The spacingthen is at a predetermined value. For example, it can be governed by thepassage of two rigid lateral strips of the required profile between thedrums.

The strands can be manufactured as tubes by extrusion and machining orcasting.

The powder strands are then assembled for making up the propellentcharge.

By way of nonrestrictive example, the manner of assembly can be bycountersinking the base (the thickest end) of the strands into a rigidbase.

To give a better understanding of the invention, by way ofnonrestrictive example, a comparison will be made between two propellentcharges of equal length, one of which is made up of strands of uniformthickness according to conventional practice, and the other of strandsof tapering thickness according to the invention. It is assumed thatthese two charges are composed of rectangular strips of powder parallelto the propellant axis.

These strips can be flat or coiled. The charge may possibly be made upof a single spirally coiled strip.

As seen in FIGS. 1 and 2 the propulsive charge comprises a plurality offlat powder bands 2 which are embedded in an inhibitor plate 1. Thegenerated combustion gas fiows towards a tube (not shown) opposite plate1 in the direction of the arrow.

The emissive surface due to the thickness of the strands is negligiblein comparison with the area of the sides of the rectangles.

By making the best use of the propellent bore, calculations show thatthe volume of powder in the case of the constant emission charge is:

lzt o wherein K is the initial propellent constriction 2 is the area ofthe inside straight section of the propellent body e is the thickness ofthe strands l is the length of the strands ,6 is the ratio ofself-contraction as previously noted.

In the case of the charge according to the invention, the thickness ofthe strands tapers over a length b.

The condition set forth in Equation 1 becomes giving an upper limit tob.

The condition of appropriate constriction requires that the thicknessshall decrease in straight-line proportion 'to the length x according tothe relationship:

The linear character of this function shows that the powder-makingapparatus does not present any delicate practical problem and thatsupervisory operations are simple.

The ratio b/l being designated by the value A, the maximum powder volumeis represented by the formula:

shows the possible obtainable gain by adopting a charge according to theinvention, having the same length l and the same thickness e It issimply expressed by putting the variable:

The curves in FIGS. 3 and 4, represent gains in the powder mass producedby propulsive charges according to the invention, in accordance withparameter R associated particularly to the more or less slender powderstrands. The various curves depend upon the rate of combustion of theselected powder.

FIG. 3 shows the variation of G in terms of R for various values of A.

Curves 1, 2, 3, 4, 5 and 6 correspond to the following respective valuesof 7\=0.1; 0.2; 0.3; 0.4; 0.5; and 0.6.

The curve G is bounded by the increasing values of R, since it isphysically necessary that the thickness e remains positive or zero. Thevariation V for a zero limiting thickness is represented by theexpression:

The variation of G is then represented by the curve f. whose algebraicformula is:

(418-1) (2R-l-l) 8R Curves f and g differ very slightly for R greaterthan 2.5 and are accordingly represented by a single curve.

As an application of the results obtained above, there can be examinedthe case of a propellant charge meeting the conditions hereunder, theseconditions being drawn from the antitank missile practice.

Constriction K =550 Constriction K =275 Coefficient of appropriateconstriction ,B=l.5 Maximum thickness e=0.5 mm.

Overall length l= mm.

The coefl'lcient R defined by Equation 5 equals 0.818.

For that value of R, the maximum gain possible is 15.8%. Thatcorresponds to a value of A comprised between 0.4 and 0.5. For the valueof )\=0.4, the corresponding gain is 15.5%.

The value of =0.4 will be adopted, giving appreciably the possiblemaximum gain and less than K /K The choice of A defines the length b andstrip thickness function, namely: b:60 mm.; e '=0.17 mm.

The choice of the value K for initial constriction defines the spacebetween strands along the uniform thickness section, that is i=0.5 mm.

The values b, e and i in combination with the starting hypotheses definefully the propellant charge.

This proves that it is possible to get appreciable gains of the order of15%, on the initial velocity of antitank high-performance missiles byadopting a charge according to the invention.

More specifically, calculation of the coeificient R facilitates anassessment of the impulse gain to be expected, the maximum value beingabout 17% for a propellant charge made up of strips. The mostsignificant gains are obtained for large values of 1/ 2, account beingtaken of the practical limitations of length to small thicknessescorresponding to combustion times of less than 1 second.

The principal applications of the invention are to missiles or rocketscomprising a short-combustion propellant stage, gas generators, thishowever not being restrictive of its scope.

The field covered by the invention comprises under the head of newindustrial products, propellant charges complying with the foregoingspecification, and products or systems utilizing such charges.

What is claimed is:

1. A propulsive charge comprising a plurality of spaced powder strandseach including a portion of constant thickness and a tapered portionwith a thickness de- 6 creasing progressively in the direction of gasdischarge flow, said strands being composed of a double base powder ofnitroglycerine and nitrocellulose.

2. A propulsive charge according to claim 1 wherein said strands areflat.

3. A propulsive charge as claimed in claim 1 wherein the powder strandsare constituted by cylindro-conical tubes.

4. A propulsive charge as claimed in claim 1 wherein the decrease in thethickness of the powder charge is linear.

5. A propulsive charge as claimed in claim 1 wherein said strandsprovide a constriction K in a plane perpendicular to gas flow, thethickness decreasing in straight line proportion to the length xaccording to the relation wherein B is a constant.

References Cited UNITED STATES PATENTS 3,048,112 8/1962 Shope 1492X3,109,374 11/1963 Rombel et al. 149-2X STEPHEN J. LECHERT, JR., PrimaryExaminer US. or. X.R. 149-97

